This application is based on Japanese Patent Application 2000-381842, filed on Dec. 15, 2000, the entire contents of which are incorporated herein by reference.
1) Field of the Invention
The present invention relates to semiconductor devices with quantum dots, manufacture manufacturing methods thereof, and semiconductor laser devices, and more particularly to semiconductor devices utilizing carriers injected into quantum dots, manufacture methods thereof, and semiconductor laser devices utilizing luminescence by recombination of carriers in quantum dots.
2) Description of the Related Art
Under the development of semiconductor processes, nano-meter scale crystal growth technologies and fine patterning technologies are being used for the manufacture of semiconductor devices. By utilizing growth and fine patterning technologies, the integration of semiconductor integrated circuit devices has been improved as a matter of course, and devices utilizing the effects of quantum mechanics, such as quantum well laser devices, are being used in practice.
Quantum dot structures have drawn attention as the ultimate structure based upon quantum mechanics. A quantum dot is an ultra fine structure having an energy level lower than a potential of a nearby region and being able to three-dimensionally confine carriers in an ultra fine region. Only two electrons can exist in one quantum dot at the ground level on the conduction band side. If a quantum dot is used as an active region of a laser device, interaction between electrons and holes can be made efficient. A laser device using quantum dots is expected to be a device which exceeds the limit of laser devices using a two-dimensionally extending quantum well layer, from the viewpoint of an oscillation threshold value, the temperature characteristics of the oscillation threshold value and the like. Studies of semiconductor devices utilizing quantum dots are vigorous, such as quantum dot memory devices utilizing the hole burning effects.
Techniques are known which artificially form quantum dots by using fine patterning technologies. Examples of a quantum dot forming method are: lithography with electron beams; a method of disposing quantum dots on vertices of pyramid crystals stacked on a mask pattern (GaAs tetrahedral quantum dot structures fabricated using selective area metal organic chemical vapor deposition, T. Fukui et al., Appl. Phys. Lett. 58(18), May 6, 1991); a method of disposing quantum dots on vertices of quadrilateral pyramids formed under a mask pattern; a method utilizing initial lateral growth of crystals on a slanted substrate; a method utilizing atom manipulation based upon STM (scanning tunneling microscopy); and the like. These methods have the common aspect that semiconductor materials are artificially processed. These methods are therefore advantageous in that the position of each quantum dot can be controlled freely.
Another method of forming quantum dots by themselves is known. The phenomenon that quantum dots are formed by themselves is called self-organization. Specifically, a semiconductor layer is formed through vapor phase epitaxial growth under the specific conditions of lattice mismatch. In this case, not a film which two-dimensionally and uniformly extends on an underlying surface but a three-dimensional fine structure (quantum dot structure) is formed by itself. With this method, as compared to artificial fine patterning, a quantum dot structure can be formed in which quantum dots are distributed at a higher density and each quantum dot has a high quality.
The best known one of self-organization of quantum dots is the Stranski-Krastanov mode (SK mode). During the growth in the SK mode, a two-dimensionally extending thin film (wetting layer) is grown initially on an underlying surface, and as source material continues to be supplied, quantum dots are formed by themselves. The quantum dots formed in the SK mode are buried in a quantum well layer so that the wavelength of luminescence of quantum dots can be controlled. Quantum dots having a uniform size can be formed by the SK mode.
Although the progress of quantum dot forming technologies is remarkable, some problems of application of a quantum dot structure to semiconductor devices are becoming distinct. One of them is a low efficiency of injecting carriers in quantum dots, and another is that an efficiency of injecting carriers to the ground level is lowered by the phonon bottleneck phenomenon.
With reference to FIG. 1B, the reason why the carrier injection efficiency of quantum dots is low will be described. FIG. 1B is a cross sectional view showing an example of a conventional quantum dot structure. On the surface of a semiconductor substrate 1 having an n-type conductivity, a plurality of quantum dots 2 are distributed dispersedly. A semiconductor layer 3 having a p-type conductivity is formed on the surface of the semiconductor substrate 1, covering the surfaces of the quantum dots 2. As a forward voltage existing between the semiconductor substrate 1 and semiconductor layer 3 is applied, electrons 10a in the n-type semiconductor substrate 1 and holes 11a in the p-type semiconductor layer 2 are injected into the quantum dots 2.
However, since the quantum dots are distributed dispersedly, some electron 10c and hole 11c are transported to the p-type semiconductor layer 3 and n-type semiconductor substrate 1 without being injected into the quantum dot 2. Some electron 10b and hole 11b are recombined in a region other than the quantum dots. Therefore, only a portion of carriers contributing to the current is injected to the quantum dots 2. If such quantum dots are used for a semiconductor laser device, a lowered carrier injection efficiency of quantum dots results in lower luminescence efficiency.
The photon bottleneck phenomenon is the phenomenon that relaxation of carriers to a discrete level (transition from a higher level to a lower level) is suppressed by the conservation of energy. Quantum dots have a delta state density function so that optical phonons are related to carrier relaxation and carrier relaxation becomes difficult to occur. It is reported that carrier relaxation of quantum dots is slower than that in a quantum well layer because of the phonon bottleneck phenomenon (for example, Physical Review B54, R5243, 1996).
It is an object of the present invention to provide a semiconductor device and its manufacture method capable of preventing the carrier injection efficiency of quantum dots from being lowered.
It is another object of the present invention to provide a semiconductor device and its manufacture method capable of mitigating the difficulty of carrier relaxation by the phonon bottleneck phenomenon.
It is another object of the present invention to provide a semiconductor laser device having high luminescence efficiency by raising the carrier injection efficiency of quantum dots.
According to one aspect of the present invention, there is provided a semiconductor device comprising: a substrate comprising a first semiconductor and having a principal surface; a plurality of quantum dots distributed dispersedly on the principal surface; a cover layer comprising a second semiconductor and formed on a plane on which the quantum dots are distributed; and a barrier layer comprising insulator or third semiconductor having a band gap wider than band gaps of the first and second semiconductors and disposed on the plane on which the quantum dots are distributed and at least in a partial area of an area not disposed with the quantum dots.
While current flows between the substrate and cover layer, carriers cannot pass through the barrier layer so that the injection efficiency of carriers into the quantum dots can be improved.
According to another aspect of the present invention, there is provided a method of manufacturing a semiconductor device, comprising the steps of: forming quantum dots distributed dispersedly on a principal surface of semiconductor; forming a first layer comprising Al-containing compound semiconductor on the principal surface in an area not disposed with the quantum dots; covering the quantum dots and the first layer with a second layer comprising semiconductor; exposing side faces of the first layer; and oxidizing the first layer from the exposed side faces.
Since the oxidized first layer exhibits electrical insulation, the oxidized first layer functions as the barrier layer so that the injection efficiency of carriers into the quantum dots can be improved.
According to another aspect of the present invention, there is provided a semiconductor laser device comprising: a substrate comprising first semiconductor of a first conductivity type; a first separation confinement hetero layer formed on a surface of the substrate; a plurality of quantum dots distributed dispersedly on the first separation confinement hetero layer; a second separation confinement hetero layer formed on a plane on which the quantum dots are distributed; a barrier layer disposed between the first and second separation confinement hetero layers, on the plane on which the quantum dots are distributed and at least in a partial area of an area not disposed with the quantum dots, the barrier layer comprising insulator or semiconductor having a band gap wider than band gaps of the first and second separation confinement hetero layers; a clad layer formed on the second separation confinement hetero layer and comprising semiconductor of a second conductivity type opposite to the first conductivity type; and a pair of electrodes for applying a voltage existing between the substrate and the clad layer.
Because the barrier layer is disposed between the first and second separation confinement hetero layers, it is possible to improve the injection efficiency of carriers into the quantum dots. Luminescence efficiency can therefore be raised.